The present invention relates to a swing drive system for cranes, particularly useful on mobile lifting cranes.
A typical crane includes a lower structure that is supported on ground engaging members, an upper structure rotatably connected to the lower structure such that the upper structure can swing with respect to the lower structure about a vertical axis, and a boom pivotally mounted on the upper structure. To produce swing torque, a swing drive system exerts a moment between the upper and lower structures. This is usually accomplished by mounting a drive gear, commonly known as a pinion gear, offset from the centerline of rotation, to produce the moment. Either the lower structure or the upper structure will usually have a ring gear having teeth on a surface thereof, and the other structure will include the pinion gear that meshes with the teeth on the ring gear to provide the swing torque. If the crane is positioned such that the axis of rotation is not completely vertical (such as a mobile lift crane positioned on sloped ground), the swing drive system also has to provide a holding force to keep the upper structure from rotating when rotation is not wanted and the center of gravity of the combined crane and any load attached thereto is not at the “bottom” of its swing path.
Mobile lift cranes typically include a carbody having moveable ground engaging members, such as tires or crawlers; a rotating bed rotatably connected to the carbody such that the rotating bed can swing with respect to the ground engaging members; a boom pivotally mounted on a front portion of the rotating bed, with a load hoist line extending there from; and counterweight to help balance the crane when the crane lifts a load. Since the crane will be used in various locations, it needs to be designed so that it can be transported from one job site to the next. This usually requires that the crane be dismantled into components that are of a size and weight that they can be transported by truck within highway transportation limits. The ease with which the crane can be dismantled and set up has an impact on the total cost of using the crane. Thus, to the extent that fewer man-hours are needed to set up the crane, there is a direct advantage to the crane owner.
For some very large cranes, the torque needed to swing the crane is very large, particularly when a large load is suspended from the load hoist line. Thus the size of the ring gear and its teeth, and the size of the pinion gear, must be large enough to generate the required torque at a reasonable size ring gear diameter. In large cranes it is typical to include multiple pinion gears that each mesh with the same ring gear, so as to be able to generate the required force. However, utilizing multiple pinion gears complicates features of the crane design, particularly when it is recognized that the ring gear and pinion gear need to mesh with a tight tolerance, taking into account the needed “backlash” in the drive system. Backlash is a term that describes the amount of free play that exists between the gear teeth—the amount that one gear can turn before it starts to turn the other gear. If there is not enough backlash, the gear teeth will wear out too fast, as there will be unnecessary contact between parts of the teeth. If the backlash is too large, there will be high impact forces on the teeth when the rotating bed starts to swing, or changes its direction of swing.
Since the ring gear is not perfectly round, and since the structure making up the rotating bed (especially if it has multiple pieces that are transported separately and then pinned together at a job site) may have tolerances associated with the connection between the rotating bed and the carbody, and tolerances in the swing drive system, getting multiple pinion gears so that they can all provide torque simultaneously and with a proper amount of backlash is problematic. Of course if all of the pinion gears could be mounted to the rotating bed so that their points of contact with the ring gear formed a perfect circle, the same diameter as the ring gear, that would help lessen the problem, but this design criteria makes manufacture of the crane very expensive. The problem increases with a greater number of pinion gears. One solution is to have a thick ring gear, so that the engaged length of each gear tooth is higher, and thus more force can be transmitted through each tooth. While cutting down on the number of pinion gears, this solution increases the weight of the ring gear.
One solution is to have multiple pinion gears all connected to the rotating bed in a manner that they are independently forced into contact with the ring gear by a structure that is pivotally mounted to the rotating bed. This makes it simple to accommodate for tolerances and free play in the connection between the rotating bed and the carbody, and tolerances in the swing drive system. However, it makes for a large number of independent components that have to be disassembled, stored, transported and reassembled each time the crane is moved. Thus it would be a great advantage if a drive system could be developed that allowed such very large cranes to utilize multiple pinion gears without needing precision in the mounting of the pinion gears and minimizing the number of independent components that have to be transported and assembled to construct the crane.